Wet processing of hides and skins are conventionally done in pit, vat, paddle and wooden drum. While using pit and vat, the hides/skins are put into the process liquor and handled manually. It has prompted the tanners to use paddle to impart the agitation in the to liquor by rotation of the blades of the paddle using motor and gearbox, thereby providing mechanical agitation to the stock to be processed. Much better effect of the desired mechanical action under dynamic condition on the hide/skin to be processed is however provided in a tanning drum, which has been in use since 1903. Rotational movement of the drum ensures that the skins/hides are first lifted up by the shelves or pegs and then allowed to fall down under gravity to the processing liquor, resulting in considerable mechanical action. As reported by Sharphouse (Leather Technician's Hand book—year 1971, Appendix 7 Pages 543 to 547 published by Leather producer's association) and K. T. Sarkar author and publisher (Theory and practice of leather manufactures—5th edition, year 1997—pages 732-761) this conventional drumming is associated with the following limitations.
a) It involves the possibility of damage of the sides/skins under processing, by scrapping, scuffing, knotting, shape distortion, wrinkling grain pebbling, loosening of fiber structure or loss in area.
b) It is associated with difficulty in thorough cleaning of the stock, which lets out lot of water adding to the effluent disposal problem.
c) It involves the inconvenience in opening the drum for unloading and other intermittent checking.
d) Since the float of the drum cannot go beyond the axle level, the capacity utilization is very low to the tune of only 35% of the volume of the drum.
e) Moreover, the turbulence generated in a tanning bath due to the rotational movement of the device results not only in reduced diffusion of chemicals, but also in enhanced friction among the surfaces of the skins/hides, thereby contributing to the degradation of the quality of the final leather.f) To process in short float as a measure against effluent problems, which requires strengthening of drum drives, a higher motor power is required for safe and efficient running.g) Normally, the required starting torque, which plays an important role in the selection of motor for any application, is very high to the tune of 3 times of the running torque due to the following reasons.1. Concentrated load of the hide/skin2. Self-weight of the wooden tanning drum,3. Friction because of the journal bearings and4. The possible spur gear errors.
It is thus obvious that the motors used for running the drums are usually oversized to take care of starting torque, resulting in poor efficiency under normal running condition.
The above limitations have prompted the researchers to explore better options for leather processing device. Tombetti et al (Abstract published in Journal of American Leather Chemists Association, 75, 62, 1980) developed a pressure vessel for leather tanning having a rotating inner drum (2) divided into three perforated compartments, each having a door (6), housed in a stationary exterior casing (1) provided with a door (7) and an inlet for chemical and water housed on a frame assembly (3), the said inner drum (2) being rotated with the help of main shaft (4), held by two bearings and housing (5) and connected to a power transmission assembly source (8), the pressure inside the inner drum (2) being generated with the help of a pneumatic system (9).
It has been possible to enhance the capacity utilization to about 60% due to the provision of plurality of compartments provided in the pressure vessel. Moreover, the distribution of load, while the vessel is in motion ensures that the starting torque required for running the device is around 2 times of the running torque, thereby suggesting economy in energy consumption. Nevertheless, the pressure vessel is also associated with the following limitations. Association of moving parts with the inner vessel, which is connected to a rotating main shaft, has greater implication of wear and tear, that is likely to result in leakage of air, thereby causing loss of pressure inside the vessel. The stationary outer shell does not provide any option for automatic unloading of the leathers, which have to be taken out manually, thereby implying more labor as well as time.
Our copending Indian patent application No. 791/DEL/2000 has been an attempt towards obviating the aforesaid limitations by way of providing an improved tanning device comprising a perforated inner vessel (15) having shelves more than one (17) and a door (16) the said inner vessel (15) being housed on non-corrosive chemical resistant rollers (18) fixed on the inner wall of the outer vessel (22) made up of a non-corrosive material capable of withstanding pressure of minimum 5 Kg/cm2, and the said outer vessel (22) being provided with a leak-proof packing (21) and dome shaped caps (20) on either side, the said outer shell being provided with inlet for chemicals and water (23), conventional regulated temperature device (24), discharge for liquid (26), a pressure nozzle (25) connected to a compressor and regulator externally (not shown in the drawing) for developing air pressure inside the vessel, the entire oscillating exterior shell (10) along with all accessories defined herein before being placed on center shaft (not shown in the drawing) being attached to the frame assembly (11) the conventional rocking mechanism (13) being provided on the outer shell and connected to the normal power transmission assembly (12) in turn connected to the mechanism for rotating action (14) thereby ensuring rocking action as well as rotational movement of the inner perforated vessel (15) with the help of non corrosive chemical resistance gear (19) fixed on the outside of the inner vessel (15).
The main advantages of this device over the known pressure vessel are the following.
1. Unlike the known pressure vessel, where only rotary motion is possible, this device provides option for both oscillatory as well as rotary motion.
2.The outer shell of the device undergoes rocking action, which enables an operator to adjust it in a convenient position so as to unload the processed stock by gravity automatically, thereby not only doing away with the extra labor associated with unloading, but also in reducing unloading time.3. Since the inner vessel is rotated by the rollers fixed to the inside of the outer shell of the device, no leakage of internal pressure is envisaged and at the same time long life is ensured for the device.4. Unlike the existing systems, the starting torque of the deice is only about 1.5 times of the running torque as against much higher values for the existing devices, thereby reducing the power requirement considerable.5. Capacity utilization of the device is around 70% as against 45% in the existing wooden drum.6. Even without opening the vessel door, the critical parameters like pH, temperature of the float can be measured only by opening the outlet.
However, the device is still constrained by the following limitations. The device provides a two-vessel system having an outer casing as well as an inner vessel, thereby escalating not only the fabrication complexities, but also cost.
An external energy source is required in the form of an air compressor to form a pneumatic system, thereby adding to cost. The device does not provide, like the conventional tanning drums, any option for automatic mixing of chemicals inside the drum, thereby implying the requirement of proper mixing of chemicals.